1. Field of the Invention
The present invention relates to an ink jet head in which a plurality of ink chambers are formed in a cavity plate and are covered with a sheet material, and piezoelectric elements are disposed on the sheet material so as to correspond to the ink chambers, respectively. More particularly, the present invention relates to an ink jet head having a predetermined relation between a width of the ink chamber and a width of the piezoelectric element disposed on the ink chamber, whereby it can maintain the speed of ink ejecting from the ink chamber within an allowable range without lowering it even if the piezoelectric element is arranged in a position displaced from a proper position on the corresponding ink chamber.
2. Description of Related Art
Heretofore, regarding an ink jet head which ejects ink droplets via a nozzle from a selected ink chamber upon application of a driving voltage to a piezoelectric element mounted on the ink chamber, a number of researches have been made on a relation between the speed of the ink droplet when ejected from the nozzle and a pulse width of the driving voltage to be applied to the piezoelectric element. It is generally well known that the ejecting speed of the ink droplet ejected from the nozzle varies periodically according to the width of pulses of the driving voltage to be applied to the piezoelectric element.
For example, in the case where a driving voltage with various pulse widths is applied to a piezoelectric element to press a selected one of ink chambers in an ink jet head thereby to eject an ink droplet from a nozzle of the ink chamber, the relation shown by a curved line S in FIG. 6 exists between the ejecting speed of the ink droplet and the pulse widths P of the driving voltage. At this time, if the time needed for a pressure wave of ink, which is generated when the ink chamber is pressed, to travel by a length of the ink chamber is considered as T, the first maximum point K1 of the curved line S (the left maximum point in FIG. 6) indicates that the pulse width P of the driving voltage corresponds to the time T and the second maximum point K2 (the right maximum point in FIG. 6) indicates that the pulse width P corresponds to the time 3T which is three times the time T.
In conventional ink jet heads, therefore, such the pulse widths P of a driving voltage for driving each piezoelectric element are selectively set based on the above mentioned relationship between the ejecting speed of an ink droplet and the pulse width P of the driving voltage to be applied to the piezoelectric element.
Meanwhile, the time T needed for a pressure wave of the ink in the ink chamber when pressed to travel by the length of the ink chamber is defined by an equation; T=L/(.sqroot.Ev/.rho.), wherein "L" represents a length of an ink chamber (see FIG. 1), Ev represents apparent volume modulus of the ink in the ink chamber, and .rho. represents the density of the ink. It is noted that the volume modulus Ev changes according to an amount of deformation of the ink chamber when pressed, namely, an amount of deformation of each wall forming the ink chamber in the cavity plate and a sheet material on which the piezoelectric element is mounted in contact with it. This volume modulus Ev has a property of becoming smaller as the deformation amount of each ink chamber wall and that of the sheet material are larger.
In particular, the irregular deformation of the sheet material (a vibrating sheet) on which the piezoelectric element is mounted, which is caused by application of a driving voltage to the piezoelectric element, may largely affect the volume modulus Ev. This causes the change in the time T and a bad influence.
Here, the relationship between the width of a piezoelectric element arranged on the sheet material and the width of an ink chamber in a conventional ink jet head will be explained with reference to FIG. 7. FIG. 7 is an explanatory view of schematically showing the relation between the width of the piezoelectric element and the width of the ink chamber in the conventional ink jet head. In FIG. 7, an ink chamber 21 is formed in a cavity plate 20. A sheet material 22 serving as a vibrating sheet is arranged on an open (upper) plane of the ink chamber 21. Further, a piezoelectric element portion 24 formed in a piezoelectric plate 23 is arranged on the sheet material 22 so as to correspond to the ink chamber 21. With such the structure, to render the deformation of the sheet material 22 uniform in both sides of the piezoelectric element portion 24 when a driving voltage is applied to the piezoelectric element portion 24, it is preferable to dispose the piezoelectric element portion 24 in a center of the upper plane of the ink chamber 21. In other words, in the case of considering the width of the ink chamber 21 as "A" and the width of the piezoelectric element portion 24 as "B", it is desirable to form an interval of (A-B)/2 in each side of the piezoelectric element 24.
The aforesaid ink jet heads, however, are usually ordered to form an interval into about several .mu.m in each side of the piezoelectric element portion 24. When there is a small displacement among the cavity plate 20, the sheet material 22, and the piezoelectric plate 23 in assembling them, a difference occurs in the deformation amount of the sheet material 22 between both sides of the piezoelectric element portion 24.
For example, examining the relationship between the amount of displacement of the piezoelectric element portion 24 on the sheet material 22 from the center of the upper plane of the ink chamber 21 and the time T needed for a pressure wave of the ink to travel by a length of the ink chamber 21, we obtained the relation shown in FIG. 8. FIG. 8 is a graph showing the relationship between the displacement amount of the piezoelectric element portion 24 and the time T, wherein a lateral axis represents an amount (.mu.m) of displacement of the piezoelectric element portion 24 from the center in the upper plane of the ink chamber 21 and a vertical axis represents a time T (.mu.s).
As clearly from FIG. 8, it is found that the time T increases as the amount of displacement of the piezoelectric element portion 24 disposed on the sheet material 22 becomes larger. For instance, the time T is about 7.5 .mu.s when the displacement amount of the piezoelectric element portion 24 is "0", while the time T increases, specifically to about 8.4 .mu.s, when the displacement amount (represented by D) is about 35 .mu.m.
An increase of the time T means that the travelling speed of the pressure wave of the ink in the ink chamber decreases, causing a decrease in the ejecting speed of the ink when ejected from the nozzle of the ink chamber. This is explained referring to FIG. 9. FIG. 9 is a graph showing the relationship between the pulse width P of the driving voltage and the ink ejecting speed V, in which a lateral axis represents a pulse width P (.mu.s) and a vertical axis represents an ink ejecting speed V (m/s) respectively.
In FIG. 9, if the displacement amount of the piezoelectric element portion 24 is "0", i.e., the piezoelectric element portion 24 is properly placed in the center of the upper plane of the ink chamber 21, the relation between the pulse width P and the ejecting speed V is shown by the curved line S1 indicated by a solid line. If the displacement amount is D, i.e., the piezoelectric element portion 24 is placed in a position displaced by an amount of "D" from the center of the upper plane of the ink chamber 21, the relation is shown by the curved line S2 indicated by a broken line. Since the pulse width P of the driving voltage is usually determined to a predetermined pulse width, in the case that the pulse width P is 3T, the ejecting speed is "V1" in the curved line S1, while the speed is reduced to "V2" in the curved line S2.
As described above, the ink ejecting speed is reduced due to the displacement amount of the piezoelectric element portion 24, which causes unstable performance of the ink jet head. As a result thereof, it is not possible to maintain stable and high print quality.